Electrolytic capacitors are well known in the art and have been used in energy storage, transfer, and conditioning applications since the early 20th century. Conventional electrolytic capacitors are constructed from aluminum foil current collectors, both of which has an oxide layer, the anode has the thicker of the two, that acts as an insulating dielectric. The aluminum foil current collectors are held apart by a paper spacer soaked in electrolyte. The aluminum foil with the oxide layer acts as the anode and the electrolyte and other aluminum foil act as the cathode, the electrolyte providing additional free energy (electron source) compared to a traditional capacitor comprised solely of current collectors separated by an insulating dielectric layer. Electrolytic capacitors are generally formed in a layered structure, then rolled and packaged in a cylindrical canister that provides leads electrically connected to the anode and cathode respectively.
A common alternative electrolytic capacitor design uses sintered tantalum (creating a porous surface with a large surface area) with an insulating oxide layer as the anode and the liquid electrolyte acting as the cathode (electrically connected to the outer canister). In this type of capacitor, the electrolyte is both a current collector and source of electrons.
Still another alternative is a polymer electrolytic capacitor, wherein a solid electrolyte is used instead of a liquid electrolyte. The solid electrolyte is impregnated in the separator sheet and provides greater physical stability and reliability due to the lack of a liquid component.
Electrolytic capacitors are distinguishable from electric double-layer capacitors (ELDCs), also known as supercapacitors or ultracapacitors. ELDCs are electrochemical capacitors that use a nanoporous (high-surface per unit volume) material, such as activated carbon, rather than a more traditional insulating dielectric. They still use a stacked configuration between current collectors, though the adjacent current collectors may in fact be part of the same rolled substrate. But between the current collectors are layers of nanoporous materials and a source of charge carriers (such as an electrolyte) with a thin separator to create the double-layer effect. So, while an ELDC contains an electrolyte, it is not an dielectric electrolyte as in the electrolytic capacitor understood by those of skill in the art.
A variety of materials have been used for dielectrics, electrodes, electrolytes, and other components of both electrolytic capacitors and ELDCs. While the two types of devices may have some similar structural and electrical characteristics, the performance characteristics and electrochemical nature of ELDCs mean that there are different design parameters and similar materials may not be functionally equivalent in ELDCs versus electrolytic capacitors.
Graphene is a carbon structure that is a one-atom-thick planar sheet of sp2-bonded carbon atoms. They form a two-dimensional hexagonal crystal lattice (though it has been observed to have a tendency to roll or buckle). Graphene is the basic building block of other graphitic structures, being rolled into nanotubes, balled into fullerenes, and stacked into graphite. A graphene platelet is a small stack of graphene sheets that are generally 1-100 nanometers thick and up to 100 micrometers in diameter. Stable graphene platelets are typically at least 3 atomic layers thick and thousands of atoms across. The difference between graphite (graphene platelets are a naturally occurring component of graphite) and graphene platelet structures is a consistent coplanar orientation among the graphene platelets.
Graphene is presently being explored for use in a variety of electrical components, including ELDCs and nano-scale integrated circuit components. In ELDCs, graphene is primarily under consideration as coated electrodes or current collectors or in a variety of graphitic nanostructures to provide a nanoporous alternative to activated carbon. A nano-scale electrostatic capacitor comprised of graphene sheet electrodes and a graphene derived thin film as the insulating layer has also been proposed.